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+1
+00:00:16,280 --> 00:00:18,330
+A warm welcome to
+
+2
+00:00:18,330 --> 00:00:20,820
+the compilers and formal
+languages module?
+
+3
+00:00:20,820 --> 00:00:24,390
+My name is Christian Urban.
+Thank you for coming.
+
+4
+00:00:24,390 --> 00:00:27,644
+Compilers. I guess
+you use compilers
+
+5
+00:00:27,644 --> 00:00:31,680
+in your daily work, be it
+the C or Java compiler.
+
+6
+00:00:31,680 --> 00:00:34,245
+And you might be curious
+in what they do.
+
+7
+00:00:34,245 --> 00:00:35,700
+But you might also be
+
+8
+00:00:35,700 --> 00:00:38,130
+intimidated to look
+what they do underneath
+
+9
+00:00:38,130 --> 00:00:39,900
+the bonnet because they are
+
+10
+00:00:39,900 --> 00:00:42,520
+quite large software systems.
+
+11
+00:00:42,520 --> 00:00:46,130
+What I like to show you in
+this module is that you
+
+12
+00:00:46,130 --> 00:00:49,310
+do not need to be an
+Über hacker to implement your own
+
+13
+00:00:49,310 --> 00:00:51,305
+compiler for your
+own language, say.
+
+14
+00:00:51,305 --> 00:00:54,155
+So that will be the main
+goal of this module.
+
+15
+00:00:54,155 --> 00:00:56,210
+You will implement
+your own compiler,
+
+16
+00:00:56,210 --> 00:00:58,310
+of course with my help.
+
+17
+00:00:58,310 --> 00:01:02,360
+What I personally like
+about compilers is that
+
+18
+00:01:02,360 --> 00:01:04,580
+the subject is a
+perfect combination
+
+19
+00:01:04,580 --> 00:01:06,350
+of theory and practice.
+
+20
+00:01:06,350 --> 00:01:07,790
+I like to hack things,
+
+21
+00:01:07,790 --> 00:01:10,595
+writing actual code,
+but I also like theory.
+
+22
+00:01:10,595 --> 00:01:13,040
+I want to understand what
+my code actually does.
+
+23
+00:01:13,040 --> 00:01:17,040
+So compilers are the
+perfect subject for me.
+
+24
+00:01:18,040 --> 00:01:20,809
+Let's have a look at the details.
+
+25
+00:01:20,809 --> 00:01:23,779
+Here's an airplane
+view of a compiler.
+
+26
+00:01:23,779 --> 00:01:25,850
+On the left-hand side you can see
+
+27
+00:01:25,850 --> 00:01:28,745
+the input program a
+developer would write.
+
+28
+00:01:28,745 --> 00:01:31,955
+And on the right-hand side is
+the output of the compiler,
+
+29
+00:01:31,955 --> 00:01:36,360
+the binary code the developer
+wants to actually run.
+
+30
+00:01:36,370 --> 00:01:40,340
+What makes such a project
+actually feasible is that
+
+31
+00:01:40,340 --> 00:01:44,165
+compilers fall neatly into
+separate components.
+
+32
+00:01:44,165 --> 00:01:47,210
+So you can see the lexer and the
+
+33
+00:01:47,210 --> 00:01:48,860
+parser, which are often called the
+
+34
+00:01:48,860 --> 00:01:50,990
+front end of the compiler.
+
+35
+00:01:50,990 --> 00:01:53,000
+And the code generation is
+
+36
+00:01:53,000 --> 00:01:55,700
+called the backend
+of the compiler.
+
+37
+00:01:55,700 --> 00:01:57,620
+And it so happens
+that we will spend
+
+38
+00:01:57,620 --> 00:01:59,930
+the first five weeks
+on the lexer and
+
+39
+00:01:59,930 --> 00:02:04,970
+the parser, and the next five
+weeks on the code generator.
+
+40
+00:02:04,970 --> 00:02:09,575
+The first component of the
+compiler is the lexer.
+
+41
+00:02:09,575 --> 00:02:14,480
+The purpose of the lexer
+is to scan a flat
+
+42
+00:02:14,480 --> 00:02:16,610
+string, which the
+input program is at
+
+43
+00:02:16,610 --> 00:02:19,145
+the beginning and separate out
+
+44
+00:02:19,145 --> 00:02:21,275
+where are the words?
+
+45
+00:02:21,275 --> 00:02:23,600
+You might think
+in Western languages,
+
+46
+00:02:23,600 --> 00:02:24,920
+that is very easy:
+
+47
+00:02:24,920 --> 00:02:26,690
+you just try to
+find out where are
+
+48
+00:02:26,690 --> 00:02:28,580
+the whitespaces
+and then you know,
+
+49
+00:02:28,580 --> 00:02:31,955
+where one word stops and
+where the next word begins.
+
+50
+00:02:31,955 --> 00:02:35,300
+But, actually, computer
+languages are more
+
+51
+00:02:35,300 --> 00:02:38,180
+like ancient languages
+that you see here.
+
+52
+00:02:38,180 --> 00:02:39,860
+For example, ancient Greek.
+
+53
+00:02:39,860 --> 00:02:42,065
+The writer wrote one word
+
+54
+00:02:42,065 --> 00:02:44,915
+after the next and not
+leaving any space.
+
+55
+00:02:44,915 --> 00:02:47,930
+So for example, in this
+very simple string here
+
+56
+00:02:47,930 --> 00:02:50,960
+read n, there is no space at all.
+
+57
+00:02:50,960 --> 00:02:53,540
+But the purpose of
+the lexer is to
+
+58
+00:02:53,540 --> 00:02:56,570
+separate it into five
+different components.
+
+59
+00:02:56,570 --> 00:02:59,450
+It has to first say,
+well there is a read,
+
+60
+00:02:59,450 --> 00:03:01,595
+then there is a left parenthesis,
+
+61
+00:03:01,595 --> 00:03:04,025
+then there's an
+identifier called n,
+
+62
+00:03:04,025 --> 00:03:07,715
+then there's a right parenthesis,
+and finally a semicolon.
+
+63
+00:03:07,715 --> 00:03:11,345
+And as you can see, there
+is no space here at all.
+
+64
+00:03:11,345 --> 00:03:14,150
+And so the task of finding where
+
+65
+00:03:14,150 --> 00:03:17,705
+are the words is actually
+quite involved.
+
+66
+00:03:17,705 --> 00:03:19,460
+Also the classification is
+
+67
+00:03:19,460 --> 00:03:22,055
+sometimes not so straightforward.
+
+68
+00:03:22,055 --> 00:03:24,350
+If, for example, a writer
+
+69
+00:03:24,350 --> 00:03:26,360
+wrote "if" on its own,
+
+70
+00:03:26,360 --> 00:03:29,000
+then this should be a
+keyword classified as
+
+71
+00:03:29,000 --> 00:03:32,615
+a keyword because it's
+from the if-then-else.
+
+72
+00:03:32,615 --> 00:03:36,800
+But if the developer wrote
+something longer like "iffoo",
+
+73
+00:03:36,800 --> 00:03:38,030
+then this might just be
+
+74
+00:03:38,030 --> 00:03:39,860
+a strange variable name
+and he needs to be
+
+75
+00:03:39,860 --> 00:03:44,250
+classified as a variable name.
+
+76
+00:03:45,220 --> 00:03:50,720
+The output of the lexer 
+is a sequence of tokens.
+
+77
+00:03:50,720 --> 00:03:53,480
+These are essentially the words in
+
+78
+00:03:53,480 --> 00:03:56,405
+a sentence and their
+classification:
+
+79
+00:03:56,405 --> 00:03:59,540
+they are nouns,
+verbs or adjectives.
+
+80
+00:03:59,540 --> 00:04:02,885
+For us, of course, the
+classification would be keywords,
+
+81
+00:04:02,885 --> 00:04:06,005
+identifiers,
+operators, and so on.
+
+82
+00:04:06,005 --> 00:04:11,480
+And these tokens are the
+input for the parser,
+
+83
+00:04:11,480 --> 00:04:13,085
+the next component in
+
+84
+00:04:13,085 --> 00:04:17,240
+our compiler. The parser
+essentially takes this list
+
+85
+00:04:17,240 --> 00:04:23,300
+of tokens and transforms it
+into a abstract syntax tree.
+
+86
+00:04:23,300 --> 00:04:27,230
+That means we have now the
+sentence, we have the words.
+
+87
+00:04:27,230 --> 00:04:30,275
+We have to relate
+the words to each other
+
+88
+00:04:30,275 --> 00:04:33,680
+in order to find out what
+this sentence actually means.
+
+89
+00:04:33,680 --> 00:04:35,405
+In this case here,
+
+90
+00:04:35,405 --> 00:04:38,595
+we have to do a read...which
+variable is affected?
+
+91
+00:04:38,595 --> 00:04:45,040
+The variable n. Once we have
+the abstract syntax tree,
+
+92
+00:04:45,040 --> 00:04:49,225
+it can go to the next component,
+to the code generator.
+
+93
+00:04:49,225 --> 00:04:52,480
+Whilst it doesn't look
+like this in this picture,
+
+94
+00:04:52,480 --> 00:04:54,100
+the code generators is usually the
+
+95
+00:04:54,100 --> 00:04:56,080
+biggest part in a compiler.
+
+96
+00:04:56,080 --> 00:04:58,720
+And here we actually
+have to cut corners.
+
+97
+00:04:58,720 --> 00:05:02,470
+Instead of producing binary
+code, which can be run
+
+98
+00:05:02,470 --> 00:05:06,820
+directly on a CPU, like X86 or ARM,
+
+99
+00:05:06,820 --> 00:05:11,035
+we actually target the
+Java Virtual Machine, the JVM.
+
+100
+00:05:11,035 --> 00:05:13,600
+This is very similar
+to the Scala compiler,
+
+101
+00:05:13,600 --> 00:05:16,480
+for example, which
+produces JVM code.
+
+102
+00:05:16,480 --> 00:05:18,940
+Or, of course, also 
+the Java compiler,
+
+103
+00:05:18,940 --> 00:05:20,845
+which produces JVM code.
+
+104
+00:05:20,845 --> 00:05:23,900
+So here's a typical
+example code which we're
+
+105
+00:05:23,900 --> 00:05:27,305
+going to produce: Something
+like variable 2
+
+106
+00:05:27,305 --> 00:05:30,545
+gets allocated in the stack.
+
+107
+00:05:30,545 --> 00:05:32,390
+You subtract ten from it.
+
+108
+00:05:32,390 --> 00:05:36,050
+You test whether the
+variable is 0 and if yes,
+
+109
+00:05:36,050 --> 00:05:40,170
+you jump somewhere and otherwise
+you reload the variable.
+
+110
+00:05:41,560 --> 00:05:45,935
+Even though we cut corners
+into could generate apart.
+
+111
+00:05:45,935 --> 00:05:48,575
+By producing code for the JVM,
+
+112
+00:05:48,575 --> 00:05:51,710
+we can still obtain quite
+impressive results.
+
+113
+00:05:51,710 --> 00:05:56,225
+Here's a program with
+a cubic runtime behaviour.
+
+114
+00:05:56,225 --> 00:05:59,525
+When it has to
+calculate with 400 units,
+
+115
+00:05:59,525 --> 00:06:02,165
+it might need five
+seconds to calculate.
+
+116
+00:06:02,165 --> 00:06:05,345
+But if it  has to
+calculate with 1200 units,
+
+117
+00:06:05,345 --> 00:06:08,165
+it already needs more
+than two minutes.
+
+118
+00:06:08,165 --> 00:06:10,940
+Now these timings,
+the blue timings
+
+119
+00:06:10,940 --> 00:06:13,910
+are obtained with an
+interpreter of that program.
+
+120
+00:06:13,910 --> 00:06:16,265
+And you can see
+just next to it,
+
+121
+00:06:16,265 --> 00:06:18,050
+the red times.
+
+122
+00:06:18,050 --> 00:06:20,359
+They are obtained
+with the compiler
+
+123
+00:06:20,359 --> 00:06:23,135
+we're going to write
+in this module.
+
+124
+00:06:23,135 --> 00:06:25,100
+And there you can see, even
+
+125
+00:06:25,100 --> 00:06:29,000
+with 1200,  the times get
+hardly off the x-axis.
+
+126
+00:06:29,000 --> 00:06:30,485
+So in this instance,
+
+127
+00:06:30,485 --> 00:06:32,630
+our compiler will
+have a speed up from
+
+128
+00:06:32,630 --> 00:06:37,589
+approximately 1 million in
+comparison to the interpreter.
+
+129
+00:06:38,350 --> 00:06:42,020
+This might be a fun task
+for your spare time.
+
+130
+00:06:42,020 --> 00:06:44,480
+This is a compiler explorer,
+which allows you to
+
+131
+00:06:44,480 --> 00:06:47,060
+write C code on the
+left-hand side.
+
+132
+00:06:47,060 --> 00:06:49,115
+It shows you on the
+right-hand side
+
+133
+00:06:49,115 --> 00:06:53,270
+the machine code an
+X86 CPU can run.
+
+134
+00:06:53,270 --> 00:06:56,060
+For example, it gives
+you these color scheme and
+
+135
+00:06:56,060 --> 00:06:59,000
+says that this addition
+in the num + num in
+
+136
+00:06:59,000 --> 00:07:01,580
+the return statement,
+translates to
+
+137
+00:07:01,580 --> 00:07:02,960
+essentially an addition of
+
+138
+00:07:02,960 --> 00:07:05,930
+an register in the machine code.
+
+139
+00:07:05,930 --> 00:07:08,480
+I think this compiler
+explorer also works for
+
+140
+00:07:08,480 --> 00:07:11,495
+the Haskell language and
+also produces ARM code,
+
+141
+00:07:11,495 --> 00:07:15,245
+not just code for the X86 CPUs.
+
+142
+00:07:15,245 --> 00:07:18,950
+As an aside, I also recommend
+to watch the movie of
+
+143
+00:07:18,950 --> 00:07:20,870
+this guy because that is
+
+144
+00:07:20,870 --> 00:07:22,940
+very much like how I worked
+at the beginning
+
+145
+00:07:22,940 --> 00:07:25,355
+when implementing our compiler.
+
+146
+00:07:25,355 --> 00:07:29,300
+I wrote some code which I
+knew how it should behave.
+
+147
+00:07:29,300 --> 00:07:30,725
+And then I just had a look
+
+148
+00:07:30,725 --> 00:07:32,900
+what another compiler produces.
+
+149
+00:07:32,900 --> 00:07:37,190
+And I imitated that code
+as much as I could.
+
+150
+00:07:37,190 --> 00:07:39,380
+Such a compiler explorer
+
+151
+00:07:39,380 --> 00:07:41,375
+also exists for
+the Java language.
+
+152
+00:07:41,375 --> 00:07:42,935
+Here's one where you can write
+
+153
+00:07:42,935 --> 00:07:44,915
+Java code on the left-hand side,
+
+154
+00:07:44,915 --> 00:07:47,930
+and on the right-hand
+side you get JVM code.
+
+155
+00:07:47,930 --> 00:07:50,255
+JVM code is a byte code
+
+156
+00:07:50,255 --> 00:07:53,405
+which cannot be run
+directly by the CPU,
+
+157
+00:07:53,405 --> 00:07:55,220
+but needs the Java
+Virtual Machine
+
+158
+00:07:55,220 --> 00:07:57,170
+to essentially interpret that.
+
+159
+00:07:57,170 --> 00:07:58,760
+This means it's not
+
+160
+00:07:58,760 --> 00:08:01,235
+the most efficient way
+how to run programs - 
+
+161
+00:08:01,235 --> 00:08:02,780
+it would be much faster to
+
+162
+00:08:02,780 --> 00:08:05,285
+generate direct
+code for the CPU.
+
+163
+00:08:05,285 --> 00:08:07,700
+But by producing bytecode,
+
+164
+00:08:07,700 --> 00:08:11,435
+we still run the
+programs quite fast
+
+165
+00:08:11,435 --> 00:08:13,520
+and it also simplifies
+
+166
+00:08:13,520 --> 00:08:16,280
+a number of issues
+in our compiler.
+
+167
+00:08:16,280 --> 00:08:18,980
+One issue is about
+memory management.
+
+168
+00:08:18,980 --> 00:08:22,055
+We don't have to be concerned
+about register allocation,
+
+169
+00:08:22,055 --> 00:08:25,505
+which we would need
+to do in a real CPU.
+
+170
+00:08:25,505 --> 00:08:27,680
+This will be done by the JVM.
+
+171
+00:08:27,680 --> 00:08:29,750
+It's also much easier to produce
+
+172
+00:08:29,750 --> 00:08:33,635
+this bytecode rather than
+actual machine code.
+
+173
+00:08:33,635 --> 00:08:37,385
+I think it's now a good time
+to come to the question,
+
+174
+00:08:37,385 --> 00:08:39,950
+why on Earth studying compilers.
+
+175
+00:08:39,950 --> 00:08:42,650
+Compilers are such an
+established subject
+
+176
+00:08:42,650 --> 00:08:43,985
+in computer science.
+
+177
+00:08:43,985 --> 00:08:46,100
+Compilers do what they do.
+
+178
+00:08:46,100 --> 00:08:48,410
+Probably forrests have
+been killed by
+
+179
+00:08:48,410 --> 00:08:52,190
+all the books that have been
+published on compilers.
+
+180
+00:08:52,190 --> 00:08:56,690
+Why on Earth studying
+compilers in 2020? And even worse
+
+181
+00:08:56,690 --> 00:08:59,659
+why implementing
+your own compiler?
+
+182
+00:08:59,659 --> 00:09:02,720
+Well, a slightly humorous take on
+
+183
+00:09:02,720 --> 00:09:05,105
+that question is
+given by John Regehr,
+
+184
+00:09:05,105 --> 00:09:08,375
+who is a compiler hacker
+from the University of Utah.
+
+185
+00:09:08,375 --> 00:09:09,770
+Essentially what he says,
+
+186
+00:09:09,770 --> 00:09:12,110
+if you're a good compiler hacker,
+
+187
+00:09:12,110 --> 00:09:14,690
+you have no problems
+of finding a job.
+
+188
+00:09:14,690 --> 00:09:17,210
+He puts it as: It's effectively
+
+189
+00:09:17,210 --> 00:09:22,320
+a "Perpetual Employment Act"
+for solid compiler hackers.
+
+190
+00:09:22,990 --> 00:09:27,380
+Regehr gives two justifications
+for that statement.
+
+191
+00:09:27,380 --> 00:09:29,690
+First, he says
+hardware is getting
+
+192
+00:09:29,690 --> 00:09:32,585
+wierder, rather than
+getting clocked faster.
+
+193
+00:09:32,585 --> 00:09:34,520
+And that's definitely true.
+
+194
+00:09:34,520 --> 00:09:36,590
+My first computer many, many,
+
+195
+00:09:36,590 --> 00:09:40,040
+many years ago contained
+only a single core CPU.
+
+196
+00:09:40,040 --> 00:09:44,030
+And it was such a simple CPU
+that we wrote machine code
+
+197
+00:09:44,030 --> 00:09:46,220
+directly for that CPU in order to
+
+198
+00:09:46,220 --> 00:09:49,740
+run our programs as
+fast as possible.
+
+199
+00:09:50,260 --> 00:09:53,600
+In contrast, today, Regehr writes,
+
+200
+00:09:53,600 --> 00:09:57,005
+almost all processors are
+multi-core nowadays.
+
+201
+00:09:57,005 --> 00:09:59,870
+And it looks like there's
+an increasing asymmetry
+
+202
+00:09:59,870 --> 00:10:02,015
+in resources across cores.
+
+203
+00:10:02,015 --> 00:10:04,445
+Processors come
+with vector units,
+
+204
+00:10:04,445 --> 00:10:07,189
+crypto accelerators, etc.
+
+205
+00:10:07,189 --> 00:10:11,930
+We have TSPs, GPUs, ARM
+big,little, Xeon Phis,
+
+206
+00:10:11,930 --> 00:10:14,630
+and this only scratches the surface.
+
+207
+00:10:14,630 --> 00:10:17,255
+And that is really a
+problem for compilers,
+
+208
+00:10:17,255 --> 00:10:20,495
+because if we now
+have multi-core CPUs,
+
+209
+00:10:20,495 --> 00:10:23,180
+that means our programs need
+
+210
+00:10:23,180 --> 00:10:26,075
+to be scheduled
+over several CPUs.
+
+211
+00:10:26,075 --> 00:10:28,220
+But the developer, of course,
+
+212
+00:10:28,220 --> 00:10:30,545
+doesn't want to know
+anything about that.
+
+213
+00:10:30,545 --> 00:10:34,655
+Also, now we have more
+CPUs in a computer,
+
+214
+00:10:34,655 --> 00:10:37,400
+but they seem to also come
+with different resources.
+
+215
+00:10:37,400 --> 00:10:40,310
+So certain tasks in
+a program need to
+
+216
+00:10:40,310 --> 00:10:43,460
+be scheduled on some
+cores, but not on others.
+
+217
+00:10:43,460 --> 00:10:46,685
+We also have for a
+long time already GPUs,
+
+218
+00:10:46,685 --> 00:10:49,025
+which are highly specialized for
+
+219
+00:10:49,025 --> 00:10:53,240
+very parallel computations
+to do with graphics.
+
+220
+00:10:53,240 --> 00:10:56,015
+They at least in
+the past few years,
+
+221
+00:10:56,015 --> 00:10:59,360
+they could only do very
+special computations,
+
+222
+00:10:59,360 --> 00:11:02,255
+but very, very fast
+and highly parallel.
+
+223
+00:11:02,255 --> 00:11:05,539
+You couldn't use them for
+general-purpose calculations,
+
+224
+00:11:05,539 --> 00:11:10,205
+which you would usually
+use CPUs. Similarly, DSPs.
+
+225
+00:11:10,205 --> 00:11:14,075
+They are needed for all
+the signal processing
+
+226
+00:11:14,075 --> 00:11:16,040
+in mobile phones.
+
+227
+00:11:16,040 --> 00:11:20,780
+Without them, we just
+wouldn't have mobile phones.
+
+228
+00:11:20,780 --> 00:11:23,525
+The second reason Regehr gives is
+
+229
+00:11:23,525 --> 00:11:25,550
+that we are getting tired of
+
+230
+00:11:25,550 --> 00:11:27,620
+low-level languages and
+
+231
+00:11:27,620 --> 00:11:30,335
+their associated
+security disasters.
+
+232
+00:11:30,335 --> 00:11:32,435
+While at the beginning
+we were still
+
+233
+00:11:32,435 --> 00:11:34,760
+happy to write machine
+code directly,
+
+234
+00:11:34,760 --> 00:11:37,175
+nobody wants to do this nowadays.
+
+235
+00:11:37,175 --> 00:11:39,515
+He writes: :We want
+to write new code
+
+236
+00:11:39,515 --> 00:11:44,120
+to whatever extent possible in
+safer high-level languages.
+
+237
+00:11:44,120 --> 00:11:46,130
+Compilers are caught right
+
+238
+00:11:46,130 --> 00:11:48,365
+in the middle of these
+opposing trends:
+
+239
+00:11:48,365 --> 00:11:50,765
+one of their main jobs
+is to have bridged
+
+240
+00:11:50,765 --> 00:11:53,240
+the large and growing gap between
+
+241
+00:11:53,240 --> 00:11:55,460
+increasingly high-level languages
+
+242
+00:11:55,460 --> 00:11:58,565
+and increasingly
+wacky platforms.
+
+243
+00:11:58,565 --> 00:12:00,320
+So here you have it:
+
+244
+00:12:00,320 --> 00:12:02,750
+It's still interesting to study
+
+245
+00:12:02,750 --> 00:12:06,244
+compilers nowadays,
+especially nowadays.
+
+246
+00:12:06,244 --> 00:12:09,875
+Well, if you want to work
+on interesting problems,
+
+247
+00:12:09,875 --> 00:12:14,570
+then very often you have to
+know compilers. Here's one:
+
+248
+00:12:14,570 --> 00:12:16,940
+In the good old
+times when we were
+
+249
+00:12:16,940 --> 00:12:19,310
+still able to fly on holidays,
+
+250
+00:12:19,310 --> 00:12:23,300
+I'm sure you flew in
+a 777 Boeing airplane.
+
+251
+00:12:23,300 --> 00:12:25,850
+It's actually already
+a quite old airplane.
+
+252
+00:12:25,850 --> 00:12:28,295
+But they had the
+following problem.
+
+253
+00:12:28,295 --> 00:12:33,020
+What happens if a CPU
+calculates the wrong result?
+
+254
+00:12:33,020 --> 00:12:36,065
+And that's actually not
+such a hypothetical problem
+
+255
+00:12:36,065 --> 00:12:40,295
+because you remember
+the Pentium CPU
+
+256
+00:12:40,295 --> 00:12:43,655
+in around 2000
+contained a bug and it cost
+
+257
+00:12:43,655 --> 00:12:48,140
+a lot of money for Intel
+to replace this CPU.
+
+258
+00:12:48,140 --> 00:12:51,470
+What happens if an CPU calculates
+
+259
+00:12:51,470 --> 00:12:56,105
+the wrong result in
+some navigation data?
+
+260
+00:12:56,105 --> 00:12:58,520
+Do you just let the
+airplane crash?
+
+261
+00:12:58,520 --> 00:13:00,650
+Well, the engineers at Boeing
+
+262
+00:13:00,650 --> 00:13:02,675
+came up with the
+following solution:
+
+263
+00:13:02,675 --> 00:13:05,055
+They writing one program that
+
+264
+00:13:05,055 --> 00:13:08,690
+essentially controls
+how their airplane
+
+265
+00:13:08,690 --> 00:13:10,295
+is supposed to fly.
+
+266
+00:13:10,295 --> 00:13:13,040
+In a programming language
+called Ada that is
+
+267
+00:13:13,040 --> 00:13:15,770
+slightly obscure
+programming language
+
+268
+00:13:15,770 --> 00:13:18,650
+bat is very well-known in
+
+269
+00:13:18,650 --> 00:13:22,040
+areas where safety
+is really paramount.
+
+270
+00:13:22,040 --> 00:13:25,010
+And what they did is they
+took this one Ada program
+
+271
+00:13:25,010 --> 00:13:28,010
+and they essentially
+took three compilers,
+
+272
+00:13:28,010 --> 00:13:29,435
+independent compilers,
+
+273
+00:13:29,435 --> 00:13:32,045
+which compiled the program
+to different CPUs.
+
+274
+00:13:32,045 --> 00:13:33,815
+One CPU from Intel,
+
+275
+00:13:33,815 --> 00:13:37,235
+one CPU for Motorola,
+and one from AMD.
+
+276
+00:13:37,235 --> 00:13:38,930
+And these are quite different
+
+277
+00:13:38,930 --> 00:13:40,955
+CPUs. Also some of them
+
+278
+00:13:40,955 --> 00:13:42,755
+have quite different philosophies
+
+279
+00:13:42,755 --> 00:13:45,245
+on how they do their calculations.
+
+280
+00:13:45,245 --> 00:13:47,330
+Now what they could do is, they
+
+281
+00:13:47,330 --> 00:13:50,690
+could put these three computers
+on a single board and
+
+282
+00:13:50,690 --> 00:13:54,335
+could now run all their
+calculations in parallel.
+
+283
+00:13:54,335 --> 00:13:56,690
+One with an Intel
+CPU, one with
+
+284
+00:13:56,690 --> 00:14:00,245
+Motorola, and one with a 
+Risc computers say.
+
+285
+00:14:00,245 --> 00:14:02,795
+And then they could
+compare the results.
+
+286
+00:14:02,795 --> 00:14:05,030
+And if these results differed,
+
+287
+00:14:05,030 --> 00:14:07,940
+then they knew one CPU must have
+
+288
+00:14:07,940 --> 00:14:11,600
+calculated the wrong result
+and probably told the pilot,
+
+289
+00:14:11,600 --> 00:14:14,850
+please don't let
+that airplane crash.
+
+290
+00:14:14,950 --> 00:14:17,270
+Not just Boeing is doing
+
+291
+00:14:17,270 --> 00:14:19,355
+interesting things
+with compilers.
+
+292
+00:14:19,355 --> 00:14:22,640
+Also, Airbus in a completely
+different setting
+
+293
+00:14:22,640 --> 00:14:24,260
+is using compilers in
+
+294
+00:14:24,260 --> 00:14:26,420
+a interesting way to get
+
+295
+00:14:26,420 --> 00:14:30,195
+their airplanes up in the
+air and let them not crush.
+
+296
+00:14:30,195 --> 00:14:33,010
+In another example, I
+have friends working
+
+297
+00:14:33,010 --> 00:14:35,350
+at Facebook who work on
+
+298
+00:14:35,350 --> 00:14:37,900
+compilers to make sense
+are they are heaps
+
+299
+00:14:37,900 --> 00:14:41,470
+and heaps and heaps of
+code written in PHP,
+
+300
+00:14:41,470 --> 00:14:42,880
+which is one of the most
+
+301
+00:14:42,880 --> 00:14:44,920
+horrible languages on the planet.
+
+302
+00:14:44,920 --> 00:14:46,630
+The bottom line is,
+
+303
+00:14:46,630 --> 00:14:50,499
+compilers are still very,
+very important nowadays.
+
+304
+00:14:50,499 --> 00:14:52,810
+And for the foreseeable future,
+
+305
+00:14:52,810 --> 00:14:56,150
+compilers still need
+to be developed.
+
+306
+00:14:57,270 --> 00:15:00,235
+When one talks about
+compilers then
+
+307
+00:15:00,235 --> 00:15:01,630
+there is, of course,
+
+308
+00:15:01,630 --> 00:15:05,395
+a magic element involved. They're
+large software systems.
+
+309
+00:15:05,395 --> 00:15:07,750
+And yes, they're supposed to
+
+310
+00:15:07,750 --> 00:15:10,525
+generate a runnable binary for
+
+311
+00:15:10,525 --> 00:15:12,105
+a high-level program,
+
+312
+00:15:12,105 --> 00:15:15,230
+but they are also supposed
+to make our programs better.
+
+313
+00:15:15,230 --> 00:15:18,920
+They optimize our
+programs to run faster.
+
+314
+00:15:18,920 --> 00:15:22,610
+And there's a lot of
+"magic" involved in that.
+
+315
+00:15:22,610 --> 00:15:24,890
+Magic in inverted quotes.
+
+316
+00:15:24,890 --> 00:15:26,480
+I would like to show you one of
+
+317
+00:15:26,480 --> 00:15:29,340
+this magic in one example.
+
+318
+00:15:31,090 --> 00:15:33,110
+I hope you still have
+
+319
+00:15:33,110 --> 00:15:36,485
+fond memories of the
+PEP module last year.
+
+320
+00:15:36,485 --> 00:15:40,280
+And you might remember BF
+language, we had to look at.
+
+321
+00:15:40,280 --> 00:15:43,670
+This BF language contains
+the kind of memory and
+
+322
+00:15:43,670 --> 00:15:45,680
+the memory pointer which can
+
+323
+00:15:45,680 --> 00:15:48,140
+be moved to the left
+or to the right,
+
+324
+00:15:48,140 --> 00:15:50,270
+where an integer in
+
+325
+00:15:50,270 --> 00:15:52,925
+the memory can be either
+increased or decreased.
+
+326
+00:15:52,925 --> 00:15:55,730
+We can print
+out the content of
+
+327
+00:15:55,730 --> 00:15:59,645
+the current cell or input
+something into a cell.
+
+328
+00:15:59,645 --> 00:16:01,850
+And we have loops, and
+
+329
+00:16:01,850 --> 00:16:04,220
+everything else is
+considered a comment.
+
+330
+00:16:04,220 --> 00:16:06,290
+What's good about
+this BF language is that 
+
+331
+00:16:06,290 --> 00:16:08,180
+we don't even need a front end.
+
+332
+00:16:08,180 --> 00:16:09,920
+We can immediately start writing
+
+333
+00:16:09,920 --> 00:16:13,529
+an interpreter or a compiler.
+
+334
+00:16:15,850 --> 00:16:18,155
+Okay, I have here
+
+335
+00:16:18,155 --> 00:16:20,600
+a very straightforward
+interpreter for
+
+336
+00:16:20,600 --> 00:16:22,865
+the BF language. You
+might recognize it.
+
+337
+00:16:22,865 --> 00:16:27,120
+And I run it with a
+benchmark program, which
+
+338
+00:16:27,760 --> 00:16:30,960
+might also recognize.
+
+339
+00:16:31,560 --> 00:16:36,835
+And this will now take
+approximately ten minutes.
+
+340
+00:16:36,835 --> 00:16:39,920
+So see you in a bit.
+
+341
+00:16:40,710 --> 00:16:43,660
+Okay, this has finished now.
+
+342
+00:16:43,660 --> 00:16:45,925
+Almost took 11 minutes.
+
+343
+00:16:45,925 --> 00:16:47,410
+The question now is,
+
+344
+00:16:47,410 --> 00:16:51,820
+can we to better? 
+
+345
+00:16:51,820 --> 00:16:53,260
+Actually, it is not difficult
+to do better
+
+346
+00:16:53,260 --> 00:16:54,970
+than this simple-minded
+interpreter.
+
+347
+00:16:54,970 --> 00:16:58,690
+It is relatively
+straightforward to take
+
+348
+00:16:58,690 --> 00:17:03,520
+a BF program and generate equivalent C code.
+
+349
+00:17:03,520 --> 00:17:06,520
+This can be easily
+done by somehow
+
+350
+00:17:06,520 --> 00:17:09,490
+representing the BF memory in
+
+351
+00:17:09,490 --> 00:17:12,310
+C. We can do this
+by just an array of
+
+352
+00:17:12,310 --> 00:17:15,380
+characters and a memory pointer,
+
+353
+00:17:15,380 --> 00:17:19,265
+which points, in this case
+in the middle of this array.
+
+354
+00:17:19,265 --> 00:17:21,800
+Then it's very easy to
+
+355
+00:17:21,800 --> 00:17:24,275
+translate the movement
+of the 
+
+356
+00:17:24,275 --> 00:17:28,610
+BF memory pointer into increments
+
+357
+00:17:28,610 --> 00:17:31,520
+and decrements of
+the C pointer.
+
+358
+00:17:31,520 --> 00:17:33,050
+Similarly, if you want to increment or
+
+359
+00:17:33,050 --> 00:17:35,915
+decrement an element
+in this array,
+
+360
+00:17:35,915 --> 00:17:38,975
+you just have to look up what
+the pointer is and increment
+
+361
+00:17:38,975 --> 00:17:42,140
+and decrement what's
+under the pointer. Similarly
+
+362
+00:17:42,140 --> 00:17:43,790
+we can print out something from
+
+363
+00:17:43,790 --> 00:17:47,450
+this array and we can put
+something into this array.
+
+364
+00:17:47,450 --> 00:17:49,610
+What is great is that the loops
+
+365
+00:17:49,610 --> 00:17:51,530
+from the BF language directly
+
+366
+00:17:51,530 --> 00:17:55,580
+translate into while
+loops of the C language.
+
+367
+00:17:55,580 --> 00:17:58,100
+We essentially have to check is
+
+368
+00:17:58,100 --> 00:18:02,450
+the memory pointer pointing
+to a field that is 0.
+
+369
+00:18:02,450 --> 00:18:03,950
+Then we exit the loop,
+
+370
+00:18:03,950 --> 00:18:05,719
+and otherwise we continue.
+
+371
+00:18:05,719 --> 00:18:09,575
+And that can be done
+nicely in C, like so.
+
+372
+00:18:09,575 --> 00:18:12,995
+And everything else is
+again, just a comment.
+
+373
+00:18:12,995 --> 00:18:16,445
+So I have implemented
+this translation for you.
+
+374
+00:18:16,445 --> 00:18:19,700
+Remember this was the BF
+program for the Mandelbrot Set.
+
+375
+00:18:19,700 --> 00:18:23,690
+The equivalent C code
+would look like this.
+
+376
+00:18:23,690 --> 00:18:27,110
+So you can see at the beginning
+is this generation of
+
+377
+00:18:27,110 --> 00:18:30,140
+the BF memory represented
+
+378
+00:18:30,140 --> 00:18:32,345
+as an array and the
+memory pointer.
+
+379
+00:18:32,345 --> 00:18:34,550
+And then inside there are lots
+
+380
+00:18:34,550 --> 00:18:36,770
+of increments and decrements
+
+381
+00:18:36,770 --> 00:18:41,915
+of pointers and also
+contents of this array.
+
+382
+00:18:41,915 --> 00:18:45,199
+Now fingers crossed that this
+is the correct translation.
+
+383
+00:18:45,199 --> 00:18:48,125
+And I can also run this
+
+384
+00:18:48,125 --> 00:18:52,805
+and you should see that it runs
+substantially faster.
+
+385
+00:18:52,805 --> 00:18:55,880
+I'm using now my GCC on
+
+386
+00:18:55,880 --> 00:19:01,040
+my computer to generate an
+executable for the C program.
+
+387
+00:19:01,040 --> 00:19:04,295
+And it should run for
+approximately 20 seconds.
+
+388
+00:19:04,295 --> 00:19:07,620
+So let's just wait for that.
+
+389
+00:19:11,430 --> 00:19:14,950
+Okay. What is important to note
+
+390
+00:19:14,950 --> 00:19:19,885
+here is that I'm running GCC,
+
+391
+00:19:19,885 --> 00:19:22,450
+this the option -O0.
+
+392
+00:19:22,450 --> 00:19:25,600
+That means I tell GCC to
+
+393
+00:19:25,600 --> 00:19:28,840
+generate a binary which I
+can run as you can see.
+
+394
+00:19:28,840 --> 00:19:31,810
+But don't apply any optimization.
+
+395
+00:19:31,810 --> 00:19:38,395
+Keep this in mind.
+As a second try,
+
+396
+00:19:38,395 --> 00:19:42,595
+of course, I can try to now
+generate a better C program.
+
+397
+00:19:42,595 --> 00:19:46,060
+And as you'll remember from
+the PEP course, it can,
+
+398
+00:19:46,060 --> 00:19:50,095
+for example, combine
+several steps
+
+399
+00:19:50,095 --> 00:19:51,670
+going to the right of the memory
+
+400
+00:19:51,670 --> 00:19:53,310
+pointer or to the left.
+
+401
+00:19:53,310 --> 00:19:55,280
+We can combine that into
+
+402
+00:19:55,280 --> 00:19:58,760
+one single increment or
+decrement of not just one,
+
+403
+00:19:58,760 --> 00:20:00,050
+but of like n,
+
+404
+00:20:00,050 --> 00:20:02,570
+where n is greater
+than 1. Similarly, 
+
+405
+00:20:02,570 --> 00:20:06,980
+if we increment or decrement
+the content of this array,
+
+406
+00:20:06,980 --> 00:20:09,740
+we can do this in one
+big step by incrementing
+
+407
+00:20:09,740 --> 00:20:12,710
+that by not just one
+and increment by one,
+
+408
+00:20:12,710 --> 00:20:15,635
+but increment and decrement
+by bigger numbers.
+
+409
+00:20:15,635 --> 00:20:18,870
+Everything else stays the same.
+
+410
+00:20:20,830 --> 00:20:23,270
+Again, I have implemented that
+
+411
+00:20:23,270 --> 00:20:24,950
+for you and you can see now
+
+412
+00:20:24,950 --> 00:20:26,835
+the C program moves
+
+413
+00:20:26,835 --> 00:20:30,455
+the memory pointer and bigger
+chunks and also increases,
+
+414
+00:20:30,455 --> 00:20:32,810
+for example, here, memory content
+
+415
+00:20:32,810 --> 00:20:35,555
+by 15 than just by 1.
+
+416
+00:20:35,555 --> 00:20:38,520
+Now let's run this program.
+
+417
+00:20:38,530 --> 00:20:40,760
+Again, I use GCC
+
+418
+00:20:40,760 --> 00:20:45,350
+to compile the
+C program and run it.
+
+419
+00:20:45,350 --> 00:20:49,025
+And again, I made sure
+that it only runs this with
+
+420
+00:20:49,025 --> 00:20:51,530
+no optimizations switched on.
+
+421
+00:20:51,530 --> 00:20:54,050
+So there's minus O0.
+
+422
+00:20:54,050 --> 00:20:56,090
+And you can see
+it's now down from
+
+423
+00:20:56,090 --> 00:20:59,990
+20 seconds to just 6 seconds.
+
+424
+00:20:59,990 --> 00:21:06,065
+I show you, the GCC is
+called with -O0.
+
+425
+00:21:06,065 --> 00:21:08,990
+So this reduction
+in time is purely
+
+426
+00:21:08,990 --> 00:21:12,755
+because I produced better C code,
+
+427
+00:21:12,755 --> 00:21:17,220
+which the compiler then just
+transformed into a binary.
+
+428
+00:21:18,910 --> 00:21:22,055
+So far there's
+nothing interesting.
+
+429
+00:21:22,055 --> 00:21:25,385
+We used in the first instance
+
+430
+00:21:25,385 --> 00:21:29,240
+single increments and use GCC with
+
+431
+00:21:29,240 --> 00:21:31,700
+O0 to not introduce
+
+432
+00:21:31,700 --> 00:21:35,255
+any optimizations and run
+essentially 20 seconds.
+
+433
+00:21:35,255 --> 00:21:37,880
+If we then increment
+
+434
+00:21:37,880 --> 00:21:40,895
+in bigger chunks or
+decrement in bigger chunks,
+
+435
+00:21:40,895 --> 00:21:45,380
+use again GCC with -O0,
+
+436
+00:21:45,380 --> 00:21:50,030
+then it runs in approximately
+five to six seconds.
+
+437
+00:21:50,030 --> 00:21:51,950
+Now let me do the following:
+
+438
+00:21:51,950 --> 00:21:55,430
+I take the first program which
+
+439
+00:21:55,430 --> 00:21:58,070
+increments everything
+in a single steps
+
+440
+00:21:58,070 --> 00:22:00,185
+or decremented in single steps.
+
+441
+00:22:00,185 --> 00:22:04,835
+But now I use the full
+capacity of the GCC compiler
+
+442
+00:22:04,835 --> 00:22:06,560
+and I tell it,
+
+443
+00:22:06,560 --> 00:22:11,370
+please do introduce some
+optimizations as you want.
+
+444
+00:22:11,590 --> 00:22:15,799
+And I'm now running
+exactly the same program...
+
+445
+00:22:15,799 --> 00:22:17,870
+just the GCC compiler will
+
+446
+00:22:17,870 --> 00:22:22,325
+now have the optimizations
+switched on.
+
+447
+00:22:22,325 --> 00:22:24,380
+Let's see what happens.
+
+448
+00:22:24,380 --> 00:22:27,320
+One first needs to compile it.
+
+449
+00:22:27,320 --> 00:22:29,795
+And that takes a little while.
+
+450
+00:22:29,795 --> 00:22:32,000
+Okay, this has now
+finished and also
+
+451
+00:22:32,000 --> 00:22:34,115
+the calculation of the
+picture has finished.
+
+452
+00:22:34,115 --> 00:22:35,960
+And as you can see, it took
+
+453
+00:22:35,960 --> 00:22:38,510
+approximately eight
+seconds to calculate.
+
+454
+00:22:38,510 --> 00:22:41,645
+That is down from
+approximately 20 seconds.
+
+455
+00:22:41,645 --> 00:22:46,040
+So the difference from
+switching from -O0 to
+
+456
+00:22:46,040 --> 00:22:51,935
+-O3 meant that now
+the program runs almost as
+
+457
+00:22:51,935 --> 00:22:54,800
+fast as where I by hand
+
+458
+00:22:54,800 --> 00:22:58,610
+combined several steps
+into a big chunk.
+
+459
+00:22:58,610 --> 00:23:00,170
+That is essentially what
+
+460
+00:23:00,170 --> 00:23:03,485
+the GCC compiler
+found out on its own.
+
+461
+00:23:03,485 --> 00:23:05,840
+That rather than jumping
+
+462
+00:23:05,840 --> 00:23:08,465
+in just single increments by one,
+
+463
+00:23:08,465 --> 00:23:11,510
+they can be all combined
+into bigger chunks.
+
+464
+00:23:11,510 --> 00:23:16,595
+It hasn't been as successful
+as if I do this explicitly.
+
+465
+00:23:16,595 --> 00:23:18,620
+But that is the magic that
+
+466
+00:23:18,620 --> 00:23:22,560
+the compiler essentially found
+out, that this can be done.
+
+467
+00:23:22,960 --> 00:23:25,700
+Just a quick recap of what I did.
+
+468
+00:23:25,700 --> 00:23:28,160
+I first run the program with
+
+469
+00:23:28,160 --> 00:23:31,730
+an interpreter and it took
+approximately 11 minutes.
+
+470
+00:23:31,730 --> 00:23:36,559
+Then I had a simple compiler
+generating a C program.
+
+471
+00:23:36,559 --> 00:23:40,880
+And the C compiler then had
+no optimization switched on.
+
+472
+00:23:40,880 --> 00:23:44,645
+In the C program had only
+small single increments and
+
+473
+00:23:44,645 --> 00:23:46,820
+small jumps. Then it took
+
+474
+00:23:46,820 --> 00:23:49,775
+approximately 20 seconds for
+to same program.
+
+475
+00:23:49,775 --> 00:23:52,460
+Then I had a more advanced
+compiler which does
+
+476
+00:23:52,460 --> 00:23:55,730
+big increments and
+also big jumps.
+
+477
+00:23:55,730 --> 00:23:57,470
+But again, the compiler didn't
+
+478
+00:23:57,470 --> 00:23:59,210
+introduce any optimization on its
+
+479
+00:23:59,210 --> 00:24:02,915
+own. Then it took
+approximately five seconds.
+
+480
+00:24:02,915 --> 00:24:05,210
+Then I went back to
+
+481
+00:24:05,210 --> 00:24:08,465
+the simple compiler with
+only small steps.
+
+482
+00:24:08,465 --> 00:24:10,400
+But then told the C compiler
+
+483
+00:24:10,400 --> 00:24:12,950
+to fully optimize this program.
+
+484
+00:24:12,950 --> 00:24:14,690
+And then it took more
+or less the same
+
+485
+00:24:14,690 --> 00:24:17,465
+time as the more advanced compiler.
+
+486
+00:24:17,465 --> 00:24:20,465
+I encourage you to
+look at this yourself.
+
+487
+00:24:20,465 --> 00:24:24,240
+As usual, all the programs
+are uploaded on KEATS.
+
+488
+00:24:25,090 --> 00:24:27,590
+To finish this
+introduction video,
+
+489
+00:24:27,590 --> 00:24:30,170
+let me give you an
+extremely brief overview
+
+490
+00:24:30,170 --> 00:24:32,855
+over the history of compilers.
+
+491
+00:24:32,855 --> 00:24:35,450
+While I  argued at the beginning
+
+492
+00:24:35,450 --> 00:24:38,915
+that it's interesting to
+study compilers nowadays,
+
+493
+00:24:38,915 --> 00:24:40,400
+obviously in this field,
+
+494
+00:24:40,400 --> 00:24:43,295
+we are standing on the
+shoulders of giants.
+
+495
+00:24:43,295 --> 00:24:46,520
+The field started with Turing and 
+Turing machines,
+
+496
+00:24:46,520 --> 00:24:49,130
+which were introduced in 1936.
+
+497
+00:24:49,130 --> 00:24:52,175
+Turing machines already had
+this concept of memory
+
+498
+00:24:52,175 --> 00:24:55,190
+and also programs
+of Turing machines
+
+499
+00:24:55,190 --> 00:24:58,775
+needed to be translated
+between different formalisms.
+
+500
+00:24:58,775 --> 00:25:01,100
+Regular expressions,
+which will play
+
+501
+00:25:01,100 --> 00:25:03,905
+an important role in our
+lexer of our compiler,
+
+502
+00:25:03,905 --> 00:25:06,800
+were introduced in 1956.
+
+503
+00:25:06,800 --> 00:25:10,370
+Arguably the first compiler
+was introduced in
+
+504
+00:25:10,370 --> 00:25:13,850
+1957 for a language called COBOL.
+
+505
+00:25:13,850 --> 00:25:16,550
+This was done by Grace Hopper.
+
+506
+00:25:16,550 --> 00:25:18,770
+And I recommend that
+you have and look
+
+507
+00:25:18,770 --> 00:25:20,900
+at this interview
+of Grace Hopper,
+
+508
+00:25:20,900 --> 00:25:22,130
+which shows she must have been
+
+509
+00:25:22,130 --> 00:25:24,424
+a very interesting personality.
+
+510
+00:25:24,424 --> 00:25:27,770
+But despite the
+maturity of this field,
+
+511
+00:25:27,770 --> 00:25:29,465
+it might be surprising that
+
+512
+00:25:29,465 --> 00:25:31,505
+research papers are
+still published.
+
+513
+00:25:31,505 --> 00:25:34,835
+And we will find that
+out in the module.
+
+514
+00:25:34,835 --> 00:25:37,760
+And a number of
+problems which we would
+
+515
+00:25:37,760 --> 00:25:41,270
+assume are already solved by now,
+
+516
+00:25:41,270 --> 00:25:43,730
+actually turning out
+that they are not solved.
+
+517
+00:25:43,730 --> 00:25:45,620
+Here in this blog post,
+
+518
+00:25:45,620 --> 00:25:47,825
+for example, Laurie Tratt
+
+519
+00:25:47,825 --> 00:25:49,550
+who is also from this department,
+
+520
+00:25:49,550 --> 00:25:51,740
+argued that parsing, for example,
+
+521
+00:25:51,740 --> 00:25:54,035
+isn't a solved problem yet.
+
+522
+00:25:54,035 --> 00:25:56,300
+I hope you will have as much fun
+
+523
+00:25:56,300 --> 00:25:58,130
+with this module as I have.
+
+524
+00:25:58,130 --> 00:26:00,750
+Thank you very much
+for listening.